Here we report on the development of non-hygroscopic and lead-free perovskite Cs3Cu2I5 (CCI) scintillator thin films for high-resolution x-ray imaging. The investigation is motivated by the bulk CCI crystal (density of 4.53 g/cm3), which has been reported to exhibit an ultrahigh light yield of 98,000 photons/MeV with reduced afterglow. Thin films were fabricated using two different techniques: hot-wall evaporation and casting of CCI – polymer composite materials. The films fabricated by both these techniques demonstrated high atmospheric stability. Independent of the fabrication method, the x-ray radioluminescence of undoped CCI films showed a bright blue emission at 450 nm from self-trapped exciton emission, whereas Tl-doped CCI films showed emission at 500 nm. Importantly, upon x-ray irradiation, a 100 μm thick CCI film exhibited a relatively high light output of 140% of Kodak Min-R 2000 film. The modulation transfer function (MTF(f)) was approximately 5 lp/mm at 10 % MTF for a 105 μm thick vapor deposited CCI film as well as 800 μm thick CCI – polymer composite film, measured using 70 kVp x-ray images of a tungsten slit phantom. The high air stability, nontoxicity, and high radioluminescence intensity with reduced afterglow, make CCI a potential replacement material for high-resolution, high-speed x-ray imaging.
Microcolumnar CsI:Tl scintillator screens have been the gold standard in X-ray imaging for many years due to their high density, high atomic number, and scintillation efficiency. The structured screens provide an improvement in performance by channeling the light to the detector, improving detection efficiency and spatial resolution. We have taken this concept a step further by laser-machining the CsI:Tl scintillator to provide pixels that match the detector pixels. This allows for still thicker CsI:Tl layers up to 700 μm pixelated with pitch of 100 μm to match CMOS flat panel pixels, thus improving X-ray absorption and resolution. We are investigating the applications of CMOS detectors with pixelated scintillators for imaging of bone microarchitecture on diagnostic Cone Beam CT (CBCT) systems to provide improved quantitative metrics for diagnosis of osteoporosis and osteoarthritis.
The scintillator design includes reflective coatings applied to the laser-cut grooves to improve optical isolation between pixels. Such coatings are created by atomic layer deposition (ALD), a unique approach, which permits formation of reflectors over inter-pixel grooves with aspect ratios as high as 140:1. Here we present initial results quantifying performance gains in CMOS detector resolution and their impact on the quality of bone microstructure segmentation. We demonstrate 77% gain in spatial resolution at 2 lp/mm and extension of the limiting resolution from 3 lp/mm to 4.5 lp/mm for the CMOS detector with a pixelated screen compared to a commercial sensor. In a bench-top CBCT study emulating diagnostic systems for orthopedic applications (extremity CBCT), we achieved >0.75 correlations in metrics of trabecular microarchitecture between pixelated CsI:Tl based CBCT and gold-standard micro-CT. The pixelated scintillator is expected to have significant impact for many other applications including mammography and digital radiography, where resolution and dose efficiency (DQE) of the detector are of critical importance.
High frame-rate imaging is a valuable tool for non-destructive evaluation (NDE) as well as for ballistic impact studies (terminal ballistics), in-flight projectile imaging, studies of exploding ordnance and characterization of other high-speed phenomena. Current imaging systems exist for these studies, however, none have the ability to do in-barrel characterization (in-bore ballistics) to image kinetics of the moving projectile BEFORE it exits the barrel.
The system uses an intensified high-speed CMOS camera coupled to a specially designed scintillator to serve as the X-ray detector. The X-ray source is a sequentially fired portable pulsed unit synchronized with the detector integration window and is able to acquire 3,600 frames per second (fps) with mega-pixel spatial resolution and up to 500,000 fps with reduced pixel resolution. This paper will discuss our results imaging .30 caliber bullets traveling at ~1,000 m/s while still in the barrel. Information on bullet deformation, pitch, yaw and integrity are the main goals of this experimentation. Planned future upgrades for imaging large caliber projectiles will also be discussed.
To achieve high spatial resolution required in nuclear imaging, scintillation light spread has to be controlled. This has
been traditionally achieved by introducing structures in the bulk of scintillation materials; typically by mechanical
pixelation of scintillators and fill the resultant inter-pixel gaps by reflecting materials. Mechanical pixelation however, is
accompanied by various cost and complexity issues especially for hard, brittle and hygroscopic materials. For example
LSO and LYSO, hard and brittle scintillators of interest to medical imaging community, are known to crack under thermal
and mechanical stress; the material yield drops quickly with large arrays with high aspect ratio pixels and therefore the
pixelation process cost increases.
We are utilizing a novel technique named Laser Induced Optical Barriers (LIOB) for pixelation of scintillators that
overcomes the issues associated with mechanical pixelation. In this technique, we can introduce optical barriers within the
bulk of scintillator crystals to form pixelated arrays with small pixel size and large thickness. We applied LIOB to LYSO
using a high-frequency solid-state laser. Arrays with different crystal thickness (5 to 20 mm thick), and pixel size (0.8×0.8
to 1.5×1.5 mm2) were fabricated and tested. The width of the optical barriers were controlled by fine-tuning key parameters
such as lens focal spot size and laser energy density.
Here we report on LIOB process, its optimization, and the optical crosstalk measurements using X-rays. There are
many applications that can potentially benefit from LIOB including but not limited to clinical/pre-clinical PET and SPECT
systems, and photon counting CT detectors.
Large penetration depth and weak interaction of high energy X-rays in living organisms provide a non-destructive
way to study entire volumes of organs without the need for sophisticated preparation (injection of contrast material,
radiotracer labels etc.). X-ray computed tomography (CT) is a powerful diagnostic tool allowing 3D image
reconstruction of the complete structure. Using hard X-rays in medical imaging leads to reduced dose received by
the patient. At higher energies, however, the conventional scintillators quickly become the limiting factor. They
must be thin in order to provide reasonable spatial resolution and preserve image quality. Nevertheless, insufficient
thickness introduces the need for long acquisition times due to low stopping power. To address these issues, we
synthesized a new structured scintillator to be integrated into CCD- or photodiode-based CT systems. Europiumdoped
Lu2O3 (Lu2O3:Eu) has the highest density among all known scintillators, very high absorption coefficient for X-rays and a bright red emission matching well to the quantum efficiency of the underlying CCD- and photodiode arrays. When coupled to a suitable detector, this microcolumnar scintillator significantly improves the overall
detective quantum efficiency of the detector. For the first time ever, structured and scintillating film of Lu2O3:Eu
was grown by electron-beam physical vapor deposition. A prototype sensor was produced and evaluated using both
laboratory X-ray sources as well as synchrotron radiation. Comparative performance evaluations of the newly
developed sensor versus commercial grade scintillators were conducted. Such synthesis of high density, microstructured,
scintillating coatings enables the development of high sensitivity X-ray detectors for CT applications.
Oxygen doped zinc telluride is a bright scintillator with one of the highest X-ray conversion efficiencies. These
properties make it an ideal choice for wide range of X-ray imaging applications in biology and medicine. With an
emission wavelength of 680 nm it is ideally suited for use with silicon imagers such as CCDs. In this paper we report a
new co-evaporation process where the oxygen dopant concentration in the evaporated film is controlled by simultaneous
evaporation of zinc oxide and zinc telluride charge. To date we have fabricated as large as 40 cm2 area films measuring
50 μm to 500 μm in thickness. The fabrication and characterization details of these and other films are discussed in this
paper.
Preclinical SPECT offers a powerful means to understand the molecular pathways of metabolic activity in animals.
SPECT cameras using pinhole collimators offer high resolution that is needed for visualizing small structures in
laboratory animals. One of the limitations of pinhole geometries is that increased magnification causes some rays to
travel through the scintillator detector at steep angles, introducing parallax errors due to variable depth-of-interaction
in the scintillator, especially towards the edges of the detector field of view. These parallax errors
ultimately limit the resolution of pinhole preclinical SPECT systems, especially for higher energy isotopes that can
easily penetrate through millimeters of scintillator material. A pixellated, focused-cut scintillator, with its pixels
laser-cut so that they are collinear with incoming rays, can potentially compensate for these parallax errors and thus
open up a new regime of sub-mm preclinical SPECT. We have built a 4-pinhole prototype gamma camera for
preclinical SPECT imaging, using an EMCCD camera coupled to a 3 mm thick CsI(Tl) scintillator whose pixels are
focused towards each 500 μm-diameter pinhole aperture of the four pinholes. The focused-cut scintillator was
fabricated using a laser ablation process that allows for cuts with very high aspect ratios. We present preliminary
results from our phantom experiments.
The europium-doped lutetium oxide (Lu2O3:Eu) transparent optical ceramic has excellent scintillation properties, namely
very high density (9.5 g/cm3), high effective atomic number (67.3), light output comparable to thallium-doped cesium
iodide (CsI:Tl), and emission wavelength (610 nm) for which silicon-based detectors have a very high quantum efficiency.
If microcolumnar films of this material could be fabricated, it would find widespread use in a multitude of highspeed
imaging applications. However, the high melting point of over 2400°C makes it extremely challenging to make
microcolumnar films of this material. We have recently fabricated and characterized microcolumnar films of Lu2O3:Eu.
These results are presented in this paper.
While a wide variety of new scintillators are now available, new cerium-doped lanthanide halide scintillators have shown
a strong potential to move beyond their familiar role in conventional gamma ray spectroscopy, toward fulfilling the
needs of highly demanding applications such as radioisotope identification at room temperature, homeland security, and
quantitative molecular imaging for medical diagnostics, staging and research. Despite their extraordinary advantages,
however, issues related to reliable, large volume manufacturing of these high light yield materials in a rapid and
economic manner have not been resolved or purposefully addressed. Also, if microcolumnar films of this material could
be fabricated, it would find widespread use in a multitude of high-speed imaging/nuclear medicine applications. Here
we report on synthesizing LaBr3:Ce scintillators using a thermal evaporation technique, which permits the fabrication of
high spatial resolution microcolumnar films and holds a potential to synthesize large volumes of high quality material in
a time efficient and cost effective manner. Performance evaluation of the fabricated films and their application for
SPECT imaging are also discussed.
The performance measurement of hypervelocity projectiles in flight is critical in ensuring proper projectile operation, for
designing new long-range missile systems with improved accuracy, and for assessing damage to the target upon impact
to determine the projectile's lethality. We are developing a modular, low cost, digital X-ray imaging system to measure
hypervelocity projectile parameters with high precision and to almost instantaneously map its trajectory in 3D space to
compute its pitch, yaw, displacement from its path, and velocity. The preliminary data suggest that this system can
render an accuracy of 0.25° in measuring pitch and yaw, an accuracy of 0.03" in estimating displacement from the
centerline, and a precision of ±0.0001% in measuring velocity, which is well beyond the capability of any existing
system.
We examined the spatial resolution of a columnar CsI(Tl), single-photon imaging system using an approach that
estimates the interaction position to better than the spread of the light distribution. A columnar scintillator was directly
coupled to a 512×512 electron multiplying CCD (EMCCD) camera (16 μm pixels) binned at 2×2 to sample at 32 μm
pixels. Optical photons from gamma-ray/scintillator interactions are sampled over multiple pixels. Resultant images
show clusters of signal at the original interaction site, clusters from Cs and I K x-rays up to several hundred microns
away, and clusters from collimator K x-rays. Also evident are depth-of-interaction effects which result in a broadening
of the light distribution. These effects result in a degradation of spatial and energy resolution. Cluster pixel data was
processed to better estimate the interaction position within the initial interaction cluster. Anger (centroid) estimation of
individual gamma-ray events yielded spatial resolutions better than 100 μm; a result previously achievable only with
pixellated semiconductor detector arrays. After proper calibration, depth-of-interaction (DOI) effects are corrected by
performing maximum-likelihood 3D position and energy estimation of individual gamma-ray interactions.
A novel approach to characterizing integrated optical waveguides is described in which laser light illuminates the surface normal of the waveguide substrate and the transmitted light observed in the far field uniquely describes a number of physical characteristics of the waveguide. The proof of concept has been demonstrated with a HeNe laser on an ion diffused waveguide in silica glass but the technique is applicable to any integrated otpical waveguide. The advantages of this approach are that it is a great deal less expensive than conventional approaches using a Nemarski microscope or an EDAX attachment to an electron microscope and is very simple to set up. The small index variations that comprise a waveguide on a substrate are normally invisible . However, using this technique, when laser light is scanned across the waveguide, a unique pattern is seen in the far field that can be interpreted to not only determine the location of the waveguide, a unique pattern is seen in the far field that can be interpreted to not only determine the location of the waveguides but also certain physical characteristics about them.
The method of optical waveguide fabrication by ion-exchange has various advantages such as low loss, ease of fabrication, low material cost, etc. In the present paper, a package developed for the simulation of the ion-exchange process has been discussed. This simulator helps in simulating the required profile and consequent mode structure by manipulating the fabrication parameters. The complete package has been written with user orientation.
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